1. Field of the Invention
The present invention relates to a power load designed especially but not exclusively for use as a dummy antenna in radio-broadcasting.
The use of power resistors in radiofrequency applications, especially in the medium-wave range, is necessary for example to carry out tests of electrical functioning as well as measurements of performance characteristics with a radio-broadcasting transmitter. To do this , the load, which is often called a dummy antenna, must possess high stability of impedance in terms of temperature and frequency so as not to cause fluctuations in the operation of the transmitter.
There is a known way, in RF applications, of making loads with water-cooled resistive elements: certain of these loads have high resistive stability up to some megahertz but have a major drawback: they require a water-cooling circuit. Now, such a circuit is costly and may be difficult to put into application.
In other loads, the resistor is formed by means of a portion of a circuit portion in which there flows water containing sodium carbonate. Certain of these loads are provided with tuning pistons to enable the adjusting of the temperature. Such loads can be used for high power values and show high temperature stability but have the drawback of requiring two water circuits, the circuit of water containing sodium carbonate and, in addition, an ordinary water circuit designed to cool the circuit of water containing sodium carbonate.
It must be noted that there is also a known way of making resistors by the so-called expanded metal technique, which consists in making a multitude of parallel slots in a metal band without removing any material but in pushing apart the edges of the slots in such a way as to form a sort of meshwork. Resistors such as these, placed end to end, are used in fields such as electrical traction, industrial electrical heating and electrical power braking in certain trains: the loads thus constituted have high temperature stability but low stability in frequency and therefore cannot be used as dummy antennas if a wide band of operating frequencies should be desired. In this respect, it must be noted that the new radiofrequency power sources in MOSFET technology are highly sensitive to load variations owing to their very low dynamic internal impedance and that, consequently, the stability of the load must be ensured beyond the working frequencies, generally up to the third harmonic of the highest working frequency.